菜鸟教程在线编辑器

 
                <!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>菜鸟教程(runoob.com)</title>
</head>
<body>
<form action="/">
<textarea id="myTextarea" name="adr">
</textarea><br>
<input type="submit">
</form>

<p>点击按钮设置 textarea 元素在表单提交前必须填写内容。</p>

<p><strong>提示:</strong> 点击 "提交" 按钮前需要点击下面的 “点我” 按钮才能看到测试效果。</p>

<button onclick="myFunction()">点我</button>

<p id="demo"></p>

<script>
function myFunction() {
  document.getElementById("myTextarea").required = true;
  document.getElementById("demo").innerHTML = "required 属性已设置。";
}
</script>
</body>
</html>
              